NI FEATURE: THE EDITORIAL DEBATE-- PROS AND CONS
|Year : 2016 | Volume
| Issue : 3 | Page : 373--375
Lilliputian nuances of giant vestibular schwannomas
BJ Sudhir, Suresh Nair
Department of Neurosurgery, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, Kerala, India
Department of Neurosurgery, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, Kerala
|How to cite this article:|
Sudhir B J, Nair S. Lilliputian nuances of giant vestibular schwannomas.Neurol India 2016;64:373-375
|How to cite this URL:|
Sudhir B J, Nair S. Lilliputian nuances of giant vestibular schwannomas. Neurol India [serial online] 2016 [cited 2021 Jul 25 ];64:373-375
Available from: https://www.neurologyindia.com/text.asp?2016/64/3/373/181562
“It may not be amiss to briefly tell the story of this gloomy corner of neurological surgery… The cerebellopontine angle, like the fence corner of the Gettysburg battlefield, might well be called the bloody angle.”
The progress of neurosurgery as a specialty is closely linked to the operative management of vestibular schwannomas. Aptly termed by Cushing as the “the bloody angle, the gloomy corner of neurological surgery,” the cerebellopontine angle has remained a fascinating arena for neurosurgeons. The battle against vestibular schwannomas gets murkier and bloodier as the tumors attain gigantic dimensions. The dramatic improvement in surgical results in vestibular schwannoma surgery in the last few decades have been credited to an early diagnosis, improved understanding of the cerebellopontine angle anatomy and development of microsurgical techniques aided by an intraoperative cranial nerve monitoring. Recently, several large clinical series boast of a greater than ninety percent removal with a mortality rate of one percent, a far cry from the surgical results, a century ago.,,,, While a plethora of options exist for the management of small vestibular schwannomas, surgery is the unchallenged management strategy for giant vestibular schwannomas except in those patients with comorbid illnesses that increase the risk of surgery; and, in those patients who refuse surgery. Patients with tumors larger than 3 cm are seldom candidates for radiosurgery because of the need to program more than one isocenter for the radiation dose. Multiple isocenter dosing exposes the normal neural tissue to excessive amounts of radiation. Hearing preservation is rarely a factor considered in the management of very large tumors. Facial nerve preservation and reducing other neurologic complications, therefore, become the most important concerns. However, the quality of life is primarily dependent on postoperative facial nerve function and its preservation remains a challenge even for the accomplished neurosurgeon. In many large tumors, good surgical outcomes with complete tumor extirpation are obtained only at the cost of facial nerve function.
The clinical presentation of giant vestibular schwannomas is not different from those of smaller tumors. Symptoms correlate with the growth pattern of the tumor. Softer tumors tend to permeate through cisternal spaces and displace the surrounding neural structures. Firmer tumors with predominant Antoni A histopathology tend to push the neighboring structures, widening the cisternal spaces. Firmer tumors stretch the facial nerve over its surface in contrast to softer tumors which displace the nerve rather than stretch it, reflecting a less favorable outcome of facial nerve function in firm vestibular schwannomas. The lower cranial nerves are enclosed in a separate cistern yielding a good plane for dissection of these nerves off vestibular schwannomas. However if the growth pattern of the tumor causes extreme stretching of the lower cranial nerves, cough and swallowing dysfunction is to be expected. Trigeminal nerve dysfunction, predominantly sensory impairment and trigeminal neuralgia, may occasionally be present in patients with giant vestibular schwannomas. The size of the tumor may not directly correlate with raised intracranial pressure and hydrocephalus. It is unusual to find a large subset of patients with facial weakness as the major symptom at presentation. This is usually a feature of cystic vestibular schwannomas.
The retro-sigmoid corridor in the park-bench or the semi-sitting position is the preferred approach by most surgeons. Skin incisions show the same variability as do patient positioning. We always prefer the classic lazy S incision while many surgeons continue to use a straight incision. Exposing the sigmoid sinus and the sigmoid transverse junction with a partial mastoidectomy helps to extend the dural opening exactly to the edge of these sinuses to provide maximal anterior exposure. Giant vestibular schwannomas do test the patience of the surgeon to the limits in three domains: patience to achieve a good cerebellar relaxation by draining cerebrospinal fluid from the cisterns, patience to circumferentially decompress the tumor to a thin capsular rim before arachnoidal dissection, and patience to dissect the facial and cochlear nerves off the tumor. Deft and rapid centrifugal internal decompression helps in dissection of its entire circumference. The goal is to obtain a thin capsule that can be folded on itself and be used as a handle to free the tumor from the brainstem and seventh–eighth cranial nerves. Premature attempts at arachnoidal dissection before sufficient tumor decompression is an indictator of a lost battle. The internal auditory canal is opened with a cutting burr after displacing of dura and then a diamond burr is used to extend it laterally to the limits of the tumor. Tumor removal may be initiated at either the canal or cistern depending on the tumor size and its extension to the canal. As the dissection of the capsule progresses, not only is further internal decompression done, but sections of the tumor capsule are removed to allow room to deflect the capsule laterally. It may be advantageous to alternate dissection of the inferior medial capsule with dissection superiorly and medially, which defines the fifth nerve invested within its own arachnoid. The seventh nerve is usually located by reflecting the inferior and medial tumor capsule further laterally and superiorly. The seventh nerve is always anterior to the eighth nerve and has a slight grayish color which is different from the adjacent brainstem. In some cases, the seventh nerve may be displaced against the brainstem and may take a superior course before turning laterally, in which case it may lie against the fifth nerve. Dissection extends along the seventh nerve towards the internal auditory meatus. A sharp dissection is recommended near the porus due to dense adhesion of the tumor to the meatal opening. Attention is then directed back to the tumor in the internal auditory canal.
Controversies exist regarding the arachnoidal relations of vestibular schwannomas., Irrespective of whether the tumor is epi- or sub-arachnoid in origin, the essential initial step is to peel off the double layer of arachnoid from the posterior tumor surface. After reduction of the tumor volume, continued dissection of the arachnoidal fold towards the brainstem can be achieved without opening the arachnoid over the fifth and lower cranial nerves, which are in separate cisterns. In the authors' experience, it is impossible to identify an arachnoid layer between the tumor and the facio-cochlear nerves as these nerves are encased in a single cistern. The key element in performing successful surgery for a vestibular schwannoma is in understanding that the flattened facial and cochlear nerves do not have an arachnoidal layer separating them from the tumor capsule. The junction is essentially the compressed and attenuated perineurium of the vestibular nerve from which the tumor has grown. If the tumor cannot be dissected from the facial nerve easily, a sub-perineural dissection is advised. Electrophysiological monitoring and direct visual inspection using the anatomical pointers to facial nerve help in preserving neural structures.
Despite meticulous adherence to dissection principles, dense adhesions may occasionally be noted between the tumor and the brainstem necessitating a 'back out' strategy. The surgeon may find that discretion is the better part of valor, and leaving a minimal remnant of tumor adherent to the brainstem is preferable to risking neurological deficits. It is essential to achieve meticulous hemostasis if residual tumor is intentionally left behind. With the present trend of focusing on preserving the functional status and ensuring a good quality of life postoperatively, it is often a protocol to deliberately leave residual tumor in the vicinity of the facial nerve and to irridate it with gamma knife after an interval to achieve a good tumor control. Concerns have been raised about the intra-operative estimation of the actual size of residual tumor that is intentionally left behind as this may result in postoperative hemorrhage and edema, and swelling of the brainstem with resultant hydrocephalus. Hemorrhage in the operative bed is also attributed to surgical damage and engorgement of chronically compressed veins in the tumor-brainstem interface. Avoidance of significant retraction, meticulous microdisection and prompt management of postoperative hypertension will minimize this complication.
Rehabilitation after surgery for giant vestibular schwannomas needs to be addressed on a priority. Neural anastomosis and muscle transfer surgeries should be offered as part of a restorative strategy. In addition, voice and deglutition rehabilitation by a specialist also helps in early freedom from nasogastric and tracheostomy tubes.
Social and economic factors are partly responsible for a higher incidence of giant vestibular schwannomas in India. Expertise of the surgeon and the expected 'loss to follow-up' status of the patients also influence management decisions in giant vestibular schwannomas. Turel et al., from Vellore have brought forth an extensive analysis of giant vestibular schwannomas. The paper reflects the scenario in the Indian subcontinent. It is unusual to find a large subset of patients with facial weakness as the predominant symptom at presentation. This is usually a feature of cystic vestibular schwannomas. It is also interesting to find a higher incidence of unilateral abducens palsy in the subset of patients with larger tumors. As mentioned in the conclusion section, there is little need to sub-classify giant vestibular schwannomas into the two groups. Group A (4–4.9 cm) and Group B (≥5 cm) had only minor difference in outcome and complication rates. The only difference in the management strategy between these two groups is the extra time spent in volumetric decompression of the group B tumors. A higher rate of post-operative CSF rhinorrhea has been attributed to drilling of the petrous in the setting of greater erosion of the internal acoustic meatus by larger tumors. Irrespective of the tumor size, a more cautious drilling, meticulous dural closure and obliteration of the drilled bone and may subvert this complication. Patience, adherence to principles of microsurgical dissection and complication avoidance form the backbone to achieve a good outcome in surgery for giant vestibular schwannomas.
|1||Ramsden RT. The bloody angle: 100 years of acoustic neuroma surgery. J R Soc Med 1995;88:464P-8P.|
|2||Samii M, Matthies C. Management of 1000 vestibular schwannomas (acoustic neuromas): Surgical management and results with an emphasis on complications and how to avoid them. Neurosurgery 1997;40:11-21.|
|3||Nair S, Gopalakrishnan CV, Vikas V, Menon G. Techniques of facial nerve preservation in vestibular schwannoma surgery. In: Monduzzi Editoriale. International Proceedings Division. Bologna; 2011. p. 55-60.|
|4||Nair S, Gopalakrishnan CV, Vikas V, Sudhir J, Abraham M, Menon G. Acoustic neurinomas: Facial nerve preservation in large and giant tumors – A personal series of over 600 cases. In: Monduzzi Editoriale. Bologna: International Proceedings Division; 2012. p. 69-74.|
|5||Misra BK. Management of vestibular schwannomas. In: Kalangu KK, Kato Y, Dechambenoit G, editors. Essential Practice of Neurosurgery. Nagoya: Access Publishing Co. Ltd.; 2009. p. 313-27.|
|6||Kumar B, Behari S, Jaiswal AK, Jha VK, Bettaswamy G, Singh U. Large and giant vestibular schwannomas: Does cisterna magna hyperproteinorrhachia influence visual status? Acta Neurochir (Wien) 2013;155:63-9.|
|7||Ojemann RG. Retrosigmoid approach to acoustic neuroma (vestibular schwannoma). Neurosurgery 2001;48:553-8.|
|8||Kohno M, Sato H, Sora S, Miwa H, Yokoyama M. Is an acoustic neuroma an epiarachnoid or subarachnoid tumor? Neurosurgery 2011;68:1006-16.|
|9||Ohata K, Tsuyuguchi N, Morino M, Takami T, Goto T, Hakuba A, et al. A hypothesis of epiarachnoidal growth of vestibular schwannoma at the cerebello-pontine angle: Surgical importance. J Postgrad Med 2002;48:253-8.|
|10||Turel MK, D'Souza WP, Chacko AG, Rajshekhar V. Giant vestibular schwannomas: Surgical nuances influencing outcome in 179 patients. Neurol India 2016;64:478-84.|